Photovoltaic cells, also known as solar cells, are currently being used in increasing applications for the conversion of sunlight into electricity. Such cells are generally made of silicon or other semiconductor material processed to provide a p-n junction near an illuminated surface of the cell. Under sunlight illumination, such cells generate a voltage difference between the top and bottom of the cell. To use this electrical energy, electrical current paths are provided to carry sunlight-generated current from the top of the cell through the desired electrical load and back to the bottom of the cell. The normal method of providing such current paths involves the use of conducting elements or metallic gridlines on the illuminated cell surface.
Because solar cells are currently expensive to produce, one method of improving the efficiency of photovoltaic solar energy systems is to focus incident sunlight from a large area onto smaller solar cells using a solar collector which incorporates a so-called primary optical concentrator. One such concentrator is a linear Fresnel lens, such as described in U.S. Pat. No. 4,069,812 to O'Neill. Another more efficient concentrator design is described in U.S. Pat. No. 4,545,366 to O'Neill entitled "Bi-Focussed Solar Energy Concentrator." Because operation of such devices increases the radiant energy flux on the solar cell, the electrical current produced by the cell is increased many times. To efficiently carry this higher current, more and larger gridlines or conducting elements are required for the cells used in solar collectors having primary optical concentrators than in non-focussing solar collectors.
Unfortunately, because such gridlines are formed on top of the solar cell itself, these conducting elements prevent some of the sunlight from reaching the semiconductor material which converts the sunlight into electrical energy. Therefore, present photovoltaic cell designs are generally based on a tradeoff between electrical resistance losses and gridline obscuration losses, the former being minimized by large and numerous gridlines and the latter being minimized by small and fewer gridlines. Present cell designs are therefore limited in electrical conversion efficiency due to the presence of these conducting elements. Typically, 5 to 25% of the illuminated area of the cell is covered by the opaque gridlines, causing a proportional loss in electrical conversion efficiency.
Another loss associated with the present photovoltaic solar energy conversion systems occurs because of cell encapsulation. To prevent damage to the cell by moisture, dirt, and atmospheric gases, present cells use clear dielectric coatings, such as glass, over the illuminated cell surface. Unfortunately, such clear encapsulants reflect some of the incident sunlight from their front surfaces, because the index of refraction of th dielectric material is larger than the index of refraction of the surrounding air. Typically, 3-5% of the incident light is lost due to this encapsulant front-surface reflection, again proportionally reducing the electrical conversion efficiency.
It is known in the prior art to provide covers for solar cells which refract incident sunlight away from gridlines and onto active cell areas. One such structure is shown in U.S. Pat. No. 4,053,327 to Meulenberg, Jr. The cover shown in this patent is designed for use with flat-plate, one-sun cell photovoltaic solar collectors. The Meulenberg, Jr. cell cover has not proven to be commercially viable for several reasons. In typical flat-plate applications, the cell is directly exposed to severe environmental conditions, which causes the exposed surfaces of the cover to collect dirt and therefore to function poorly. For flat-plate applications, cell covers such as shown in the Meulenberg, Jr. patent are also expensive, because every portion of the energy collection area of the solar cell must be covered. Moreover, because the covers must be molded and attached to the cells, there are also significant labor costs involved in addition to the costs for the material itself.
Yet another problem which has prevented the widespread use of such cell covers is that the energy collection area for the solar cell is normally maintained in a fixed orientation (e.g., affixed to the rooftop of a home). Because the angle between the rays of the sunlight and the energy collection area of the solar cell changes continuously throughout the day and throughout the year for fixed flat-plate collectors, cell covers such as shown in the Meulenberg, Jr. patent perform poorly for most of the time, because they require perpendicular orientation to the solar rays for proper performance.
Moreover, cell covers such as described inthe Meulenberg, Jr. patent have proven inefficient for commercialization because of the different thermal expansion/contraction characteristics of the solar cell material and the cover material. Most photovoltaic cells are made of silicon, which possesses a very low thermal expansion coefficient on the order of 3.0.times.10.sup.-6 per degree F.degree.. In contradistinction, the material used in making prior art cell covers is typically glass or rigid plastic, such as acrylic, which generally has a large thermal expansion coefficient on the order of 4.5.times.10.sup.-5 per degree F.degree.. The coefficient mismatch between the cell and the cover normally causes the cover to become severely misaligned relative to the gridlines during normal operation, which involves large variations in operating temperature. Because the expansion/contraction dimensional change of the cover is normally of the same magnitude as the normal gridline spacing and gridline width dimensions, substantial misalignment occurs. such misalignment results in severe performance degradation.
Accordingly, there is presently a need for an improved solar energy collector having a primary optical concentrator which takes full advantage of solar cell cover devices.